How big a difference with 6x9 enclosures?

So I just got new rear speaker, Pioneer D series 6x9 Coaxial speakers 80rms. Being powered with at least 60rms. My old speakers were mounted in the rear 6x9 rear deck lid hole and they have those ford recycled foam looking speaker enclosures. I was wondering how they sound vs an mfd enclosure. I don't have the room under a rear deck lid for an mdf enclosure since the trunk bars touch right up to the foam thing as is. I was considering fiberglassing the outside the foam piece but was then to concerned that it's not large enough inside and making it sealed would hurt it. My other idea was mounting custom enclosure on top of the lid backed up to the corners of my car the vinyl top has a small back window with big corners... Any suggestions would be well appreciated, I want the best sound possible but if the foam things along are not much worse then a legit mdf enclosure I'll probably just leave it...